Macro and Nano-tribology of Carbon nano tubes

09.07.2024

Introduction

Macro-tribology focuses on the study of friction and wear at a large scale, typically involving macroscopic systems and components. In contrast, nanotribology is concerned with understanding the behaviors and changes in frictional interfaces at the nanoscale, where interactions occur at the level of atoms and molecules. Nanomaterials have been developed to efficiently reduce friction and wear due to their excellent physicochemical properties and lubricating potential. Among them, carbon nanotubes (CNTs), discovered in 1991, have attracted significant attention due to their high aspect ratio, exceptional mechanical strength, high thermal conductivity, and low density. The tube nanostructure of CNTs allows for rolling or sliding motion and tribofilm formation under applied stress. Additionally, CNTs exhibit a ‘sword in sheath’ phenomenon at ultra-low friction states, showing no wear or fatigue at the atomic scale, while their self-lubricating nature significantly reduces friction and wear at the macro-scale. Consequently, CNTs are recognized as crucial constituents in enhancing the tribological behavior of various material systems.

Figure-1 The tribological potential of promising carbon nanotubes and their related engineering applications.

Macroscale Tribology of CNTs

Composite coatings, bulk materials, and lubricants reinforced with CNTs have gained significant interest in tribology due to their enhanced mechanical properties, thermal conductivities, and tribological performance. CNTs can reduce friction by filling and repairing worn surfaces or by acting as nano-roller bearings to promote low rolling friction. Additionally, they can form beneficial tribofilms on worn surfaces, resulting in reduced friction (even achieving superlubricity) and high anti-wear properties. However, these benefits are highly dependent on the compatibility and homogeneous dispersion of CNTs within matrices (solid or liquid). Therefore, the choice and design of substrates, the concentration, modification, and orientation of CNTs, as well as their rational integration with other materials, play crucial roles.

Nanoscale Tribology of CNTs

Most experimental investigations of CNT tribology at the nanoscale have used techniques like atomic force microscopy (AFM), friction force microscopy (FFM), or surface force apparatus (SFA) to explore the frictional behaviors and mechanisms of CNT surfaces or CNTs as components in materials. These studies are valuable for applying CNTs in micro and macro-tribological contexts. Research has shown that the nanofriction properties of CNTs are anisotropic, with differences in friction behavior depending on whether the friction direction is parallel or perpendicular to the CNT axis. Generally, both individual and aligned CNTs exhibit lower friction along the longitudinal direction (parallel to the CNT axis) compared to the transverse direction (perpendicular to the CNT axis). This difference is due to the easier longitudinal sliding which creates a deformed dimple that moves smoothly along the tube, whereas transverse sliding causes the tube to deform and swing, resulting in greater friction dissipation. However, structural defects and surface functional groups in CNTs can increase intrinsic friction and hindered rolling. While defects generally increase friction, they can also enhance properties like fracture toughness in ceramic composites by improving interfacial friction resistance through mechanical interlocking.

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Figure-2 The illustrations depict the arc discharge (AD) CNTs, chemical vapor deposition (CVD) CNTs, and acid-functionalized CVD (FCVD) CNTs during the longitudinal sliding of the tip, accompanied by AFM images of the CNTs.

In recent years, the nano-friction properties of vertically aligned multi-walled carbon nanotube (VACNT) films and arrays have been extensively studied using AFM and FFM to enhance the macroscale application of CNT-based materials with superior tribological performance. For example, experiments on VACNT films (2 µm length and 200 nm diameter) showed that CF4-plasma modification of CNT sidewalls reduced the friction coefficient by 50% (from 0.6 to 0.3) due to decreased surface energy, which weakened adhesion between the AFM tip and VACNT surfaces. Additionally, aluminum coating on VACNT surfaces significantly reduced friction, achieving a 79% reduction in the friction coefficient (from 2.13 to 0.45) under 75 nN load. This reduction was attributed to increased lateral stiffness of the coated forest, which decreased contact areas and thus lowered friction.

Reference

[1] Zhang, F.Z., Liu, X.B., Yang, C.M., Chen, G.D., Meng, Y., Zhou, H.B. and Zhang, S.H., 2024. Insights into robust carbon nanotubes in tribology: From nano to macro. Materials Today.

[2] He, T., Chen, N., Fang, J., Cai, G., Wang, J., Chen, B. and Liang, Q., 2022. Micro/nano carbon spheres as liquid lubricant additive: Achievements and prospects. Journal of molecular liquids, 357, p.119090.

[3] Chiu, H.C., Ritz, B., Kim, S., Tosatti, E., Klinke, C. and Riedo, E., 2012. Sliding on a nanotube: Interplay of friction, deformations and structure. Advanced Materials, 24(21), pp.2879-2884.

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I am currently working as a Postgraduate Researcher at the University of Leeds, where I am actively involved in research activities. Prior to this, I successfully completed my master's degree through the renowned Erasmus Mundus joint program, specializing in Tribology and Bachelor's degree in Mechanical Engineering from VTU in Belgaum, India. Further I handle the social media pages for Tribonet and I have my youtube channel Tribo Geek.

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